Porous filter membranes are produced from many different thermoplastic materials, such as those based on cellulose derivatives, polyamides or polysulfones, that present many different pore sizes and pore size distributions, and that can be either symmetrical or asymmetrical as well as hydrophilitic or hydrophobic. EP 96 306 describes such filter membranes.
Such filter membranes are frequently brittle, have low stability under loads, and are susceptible to tearing or tear propagation, so that problems occur with their handling or their mounting into filter modules, such as filter cartridges. Modifications of the physical properties within and immediately outside of the securing area can occur particularly when filter membranes are secured in a pleated manner, e.g., in filter cartridge housings, be it through embedding in cold- or hot-setting multicomponent resins or through embedding in smelts of synthetic thermoplastics and the subsequent hardening of the sealing material. By way of example, a hydrophobization of hydrophilic membranes can thus occur. When embedding hydrophilic filter membranes, such as, e.g. nylon membranes, into hydrophobic retaining elements, such as, e.g., end caps out of polypropylene, polysulfone or polyethylene, the membrane filter edges are hydrophobized beyond the embedding region, so that this edge region, that is no longer wettable by water, contains a large number of pores which, when subjected to the bubble-point test or the pressure resistance test (integral test) of, for example, the filter cartridge, represent an air by-pass, thus making it impossible to test the filter cartridge.
In EP 96,306, the sealing of the borders of hydrophilic membrane filters is described, such a nylon filters, by means of heat-sealing polyester film that is provided on one of its sides with a solvent-free polyethylene coating as fusion adhesive. It is also mentioned that it is possible to obtain a decreased porosity at the border strips of membrane filters by compressing them mechanically, which results in a collapsing of the micro- or ultraporous filter matrix over the whole of the filter thickness.
In DE-OS 38 03 341 and EP 03 27 025, porous membrane filters are described which, on the basis of a conversion of the structure on one side of the membrane into a film-like state, present fluid-impervious spots. The liquidity of the membrane material is obtained by having steam of a solvent or of a solvent mixture for the membrane filter material act directly upon the spot to be liquified on one side of the membrane filter, so that the membrane filter material is dissolved to the desired depth. Thermal processes are also described, but all processes have the disadvantage that obtaining the one-sided sealing is technologically very difficult, that the membrane's morphology is affected, and, at least not on the side of the membrane to be coated, that no complete securing of the embedding mediums takes place.
Beside a "heat seal" and a mechanical process, EP 00 36 315 makes mention of a process in which the sensitive region of the porous filter membranes is treated by pouring with an adhesive substance. The disadvantage of this process is the insufficient temperature stability of the glued membrane region being exposed to repeated steam cycles at 134.degree. C.
All that the above-mentioned processes have in common is that in the regions to be embedded the membrane is absolutely impervious on one or on both sides, that is to say, that the surface is sealed in such a manner that hydrophobic embedding material cannot penetrate anymore into the membrane.
Patent JP-A-3012223 describes a process in which the end areas of hydrophobic hollow fiber membranes are first subjected to an oxidizing treatment, for the adhesive joining the hollow fiber membranes to adhere more firmly.
The patent JP-A-3086206 describes a process for the production of pleated hydrophobic membranes, of which the end regions are embedded in a thermoplastic synthetic material. All other areas, excepting these end regions, are soaked in a hydrophilizing agent.